Three types of technologies are currently used for fabricating microarray biochip: in situ synthesis, contact printing with spotting pins, and non-contact dispensing. Among these technologies, in situ synthesis can only be used to fabricate oligonucleotide microarrays. Contact printing with spotting pins is very simple and easily implemented; and thus, it is the most widely used technology nowadays. However, the sample volume printed for each spot depends on the physical dimensions of the spotting pins which are difficult to control, and reproducibility of the printed sample fluid volume is low. Non contact dispensing techniques provide control to fluid delivery volume and reproducibility is good as compared to contact printing with spotting pins. There is no need of contact between the dispenser and the substrate; and thus, printing speed can be much faster.
There are three types of non contact dispensing techniques, sorted according to mechanisms: microvalve control, piezoelectric jet, and thermal bubble jet. The key components for microvalve based dispensing technique include a syringe pump and a solenoid operated microvalve, such as BioJet Plus™ series developed by BioDot Company. The syringe pump is used to maintain the pressure inside the tubing between the pump and solenoid microvalve, and to aspirate sample fluid into the apparatus. Under a certain magnitude of pressure, a certain amount of fluid could be ejected through the nozzle by opening the microvalve for a certain period of time. The BioJet Plus™ series dispensers can work in two modes. In one mode, sample fluid is aspirated into the syringe, and the syringe is pushed to fill the tubing connected to the microvalve. A relative large sample volume is required, additional routine maintenance becomes necessary when changing between samples and washing the conduits. In the other mode, the conduits are filled with a certain volume of system fluid before sample fluid is actually aspirated in. The requirement on the sample fluid volume is reduced, but diffusion may be introduced on the interface between system fluid and sample fluid; and thus, it is difficult to recollect samples left. The solenoid microvalve is used to control dispensing volume. The disadvantage of BioJet Plus™ series includes: relative large sample volume or inevitable sample waste; high cost imposed by high precision syringe pump to adjust pressure; difficulty in washing due to the full filled conduit, especially under continual ejection mode; and during dispensing operation, the need to continuously propel the syringe in precise displacement to maintain pressure, and to tune the displacement finely to the decrease of the liquid volume in the conduit for constant pressure output.